The effects of various precipitation conditions on the formation of silver halide emulsions comprising tabular crystals have been A studied extensively as the said tabular grains are known in the photographic art for quite some time. As early as 1961 Berry et al. described the preparation and growth of tabular silver bromoiodide grains in Photographic Science and Engineering, Vol 5, No 6. A discussion of tabular grains appeared in Duffin, Photographic Emulsion Chemistry, Focal Press, 1966, p. 66-72. Early patent literature includes U.S. Pat. Nos. 4,063,951; 4,067,739; U.S. Pat. No. 4,150,994; 4,184,877 and 4,184,878. However the tabular grains described herein cannot be regarded as showing a high diameter to thickness ratio, commonly termed aspect ratio. In a number of U.S. Applications filed in 1981 and issued in 1984 tabular grains with high aspect ratio and their advantages in photographic applications are described as e.g. in U.S. Pat. Nos. 4,434,226; 4,439,520; 4,425,425; 4,425,426 and 4,433,048. A survey on high aspect ratio silver halide emulsions appeared in Research Disclosure, Vol 225, January 1983, Item 22534.
The above cited references on tabular grains are mainly concerned with high sensitive silver bromide or silver iodobromide emulsions.
In more recent patent applications emulsions comprising tabular crystals rich in chloride are contemplated. So precipitation conditions including ammonia as disclosed in U.S. Pat. No. 4,399,215 are resulting in rather thick tabular grains. Characteristic for the preparation of emulsions comprising tabular grains rich in chloride is the use of grain growth modifiers as e.g. in U.S. Pat. No. 4,400,463 wherein the preparation is described of a new crystallographic form of tabular silver halide grains rich in chloride by performing the precipitation in the presence of a special peptizer having a thioether linkage and an aminoazaindene growth modifier, adenine being preferred therein. Further U.S. Pat. No. 4,804,621 describes a process for preparing tabular grains rich in chloride in the presence of aminoazapyridine growth modifiers represented by a general formula of which adenine and derivatives are excluded, just as described in EP-A's 0 481 133 and 0 532 801.
In U.S. Pat. No. 4,713,323 the important role of the protective colloid used during the precipitation of emulsions comprising tabular grains rich in chloride has been disclosed: the presence in the reaction vessel of oxidized gelatin, containing less than 30 .mu.moles of methionine per gram is of crucial importance. The same conditions for gelatin in a condition for use in emulsion preparation have also been mentioned for AgBr(I) tabular grains as e.g. in U.S. Pat. No. 4,713,320. The degree of oxidation of the protective colloid can be used in order to control the tabularity of such grains. Otherwise it is generally known that the use of e.g. phthalated gelatin as a protective colloid has a strong influence on the decrease of the percentage of the total projected area represented by said tabular grains.
The protective colloid used as a binder and added in addition before coating of hydrophilic colloidal layers on a suitable support is also relevant in determining the physical properties of the resulting coated film material. Properties as e.g. dimensional stability, scratchability, curl, pressure sensitivity and sludge formation after processing are highly dependent on the choice of the protective colloid. A phenomenon like pressure sensitivity may appear as pressure marks, pressure sensitization or desensitization, wherein both the protective colloid and the coated matrix have to dissipate the energy developed by the pressure force when the coated layer is dried and deformed afterwards in packaging, before and after exposure and by processing. In the dry state pressure sensitization or desensitization may occur. Crystals having a cubic or a tabular habit are usually very sensitive to pressure sensitization.
Moreover thin coated layers by coating lower amounts of binder e.g. for rapid processing applications makes this failure become worse, especially for materials coated from layers having thin tabular silver halide grains.
Since adsorption of the protective colloid occurs at the crystal surface, development characteristics are further influenced by said protective colloid. In the wet state pressure sensitization of the film material is related with the vulnerability of the coated layers and thus with its degree of hardening or cross-linking of the binder material. This phenomenon is usually called the "roller mark" or "pressure mark" phenomenon.